The present invention pertains to a circuit arrangement for developing decoupled operating voltages for amplifier stages which may be incorporated in an integrated circuit.
When amplifier stages are coupled in cascade and derive operating voltages from a common power supply line, undesired oscillations may occur. Such oscillations may occur because the power supply line provides a feedback path by which alternating signal developed in one stage may be fed back to a preceding stage. If the cascaded stages provide a phase shift of 180 degrees and a gain greater than unity between an input point and an output point coupled together through the power supply line, the conditions for oscillation are present.
To prevent undesired oscillations of the type described above, a capacitor is often connected between a point near the beginning of the power supply line and a signal ground potential point. Such a capacitor shunts alternating signals to ground thereby reducing the amplitude of alternating signals which are present on the power supply line. This technique is not very effective where the power supply has a relatively high resistance between points coupled together through the power supply line. This latter condition is often found in an integrated circuit in which a supply voltage is applied to several cascaded amplifier stages by means of a relatively thin metalization layer, which is conventionally known in the integrated circuit arts as a power supply rail.
In discrete circuitry individual capacitors may be connected in shunt at points along the power supply line to which respective amplifier stages are connected to attenuate alternating signal on the power supply line which may be fed back from succeeding stages. However, such provisions are not well suited to integrated circuits because capacitors of sufficient value to decouple points along a power supply line to inhibit undesired oscillations at frequencies commonly encountered cannot be incorporated in the integrated circuit without an excessive use of integrated circuit area.
In integrated circuits, power supply points of individual amplifier stages may be decoupled by use of emitter-follower configured transistors. For example, such an arrangement is disclosed in U.S. Pat. No. 3,518,458 issued in the name of Camenzind. In his arrangement, the power supply point of each amplifier stage in an integrated circuit is connected to the emitter electrode of a respective emitter-follower configured transistor. The base electrode of each transistor is connected to a relatively stable voltage source such as a Zener diode. The power supply points of the individual amplifier stages tend to be decoupled because emitter-follower configured transistors exhibit a unilateral impedance transformation from a relatively high impedance at their base electrodes to a relatively low impedance at their emitter electrodes. As a result, alternating signals developed in one amplifier stage are inhibited from reaching and thereby possibly affecting the voltage source and the power supply points on the other amplifier stages. Unfortunately, the isolation afforded by emitter-follower configured transistors is limited to relatively low frequency, e.g., below 10 MHz, applications.